Conventional practice in the recovery of copper, zinc and nickel from their sulfidic ores involve subjecting the ores to a froth flotation operation to produce a concentrate of the valuable metal sulfides and to reject in the flotation tailings valueless sulfides and the silicates, aluminates, and other valueless earthy material. The concentrates are treated in various ways, depending on the metal values present. Commonly copper and nickel flotation concentrates are smelted to produce copper and nickel metal, and zinc flotation concentrates are roasted to oxide and are either reduced with carbon (coke) or are leached with acid and reduced to metal by electrowinning.
Only occasionally has it been proposed to leach sulfide concentrates directly to produce a leach solution from which the metal value can be recovered, either by direct chemical reduction or electrolytically. One such procedure, proposed in the Forward U. S. Pat. Nos. 2,576,314, 2,726,934 and 2,822,263, involves leaching nickel or copper concentrates with ammonia in the presence of air in an autoclave at high temperature and pressure. The resulting copper or nickel bearing leach solution is treated in an autoclave at high temperature and pressure with hydrogen or carbon monoxide to precipitate the nickel or copper in finely divided metal form. The Forward process has been used successfully for treating relatively high-value nickel concentrates, but has not been economically satisfactory for treating copper concentrates.
In our aforementioned co-pending applications we have described an improved process for leaching copper, nickel and zinc concentrates with an aqueous ammoniacal solution at relatively low temperatures and at pressures near atmospheric. The metal values of the leach solution are recovered by liquid ion exchange followed by electrolysis, or in any other suitable manner.
It has also been proposed to treat copper concentrates with strong (near 100%) sulfuric acid to convert the metal sulfides to sulfates, with some release of elemental sulfur. Copper in concentrates thus treated is recovered in an acid copper sulfate solution, from which it may be extracted by any of various known means. While this procedure is technically feasible, it has not thus far proved to be commercially practicable.
Except as described above, all commercial processes of which we are aware for recovery of metal values from flotation concentrates by leaching have involved roasting the concentrates to oxides (with consequent evolution of major amounts of sulfur dioxide) and leaching the resulting roasted calcine with sulfuric or other acid. Except in the treatment of zinc concentrates preparatory to electrowinning the zinc, this procedure has not been found to possess commercial advantage over conventional sulfide smelting techniques.